1. Technical Field
The present invention relates to a digital image encoding device which uses JPEG2000 as an international standard compression scheme of color still images or the like, to a digital image encoding program, to a digital image encoding method, to a digital image decoding device, to a digital image decoding program, and to a digital image decoding method.
2. Related Art
Currently, ‘JPEG (Joint Photographic Expert Group)’, which is used as an image compression format standard of color still images or the like of a digital camera is an international standard compression scheme in which a discrete cosine transform (DCT) is primarily used in combination with run length encoding or Huffman encoding. However, with ‘JPEG’, when the compression ratio is increased, there is a problem in that block-shaped encoding strain appears. Further, since ‘JPEG’ is an irreversible compression scheme, if compressed, the same image as an original image cannot be obtained.
On the contrary, a new image compression scheme ‘JPEG2000’ uses a discrete wavelet transform (DWT), instead of DCT, so that block strain does not occur even when the compression ratio increases. Further, since ‘JPEG2000’ is a reversible compression scheme, it is advantageous in that an original image can be perfectly expanded while keeping a high compression ratio. Therefore, JPEG2000 has become the most significant image format in various fields relating to digital images.
Although ‘JPEG2000’ has an excellent advantage, which cannot be obtained from ‘JPEG’, there is a problem in that the amount of information is enormous at the time of compression and expansion processes and thus the processing speed is low. Further, there is a problem in that a large memory is consumed when a processing is performed in hardware.
In order to solve the above-described problems, for example, in Japanese Unexamined Patent Application Publication No. 2004-15741, a method has been suggested in which only required bits are truncated before a bit plane so as to reduce time or power for bit plane processing. In addition, in Japanese Unexamined Patent Application Publication No. 2004-40429, a method has been suggested in which a lower bit plane substituting unit is provided next to a coefficient modeling processing unit, which performs coefficient modeling processing after the wavelet transform, so as to substitute all of the lower bit planes with ‘0’ so as to reduce the amount to be processed, thereby increasing the processing speed and reducing the amount of memory consumption.
According to the method disclosed in Japanese Unexamined Patent Application Publication No. 2004-15741, the required bits are truncated so as to truncate unused bits, thereby reducing the time or power of the bit plane processing. However, this causes a problem in that high-speed processing cannot be achieved when unused bits do not exist. Further, when this method is applied to ‘JPEG2000’, high-speed processing cannot be achieved since ‘JPEG2000’ does not process unnecessary bits at the beginning.
Further, according to the method disclosed in Japanese Unexamined Patent Application Publication No. 2004-40429, all of the lower bit planes are substituted with ‘0’ so as to reduce the amount to be processed, thereby increasing processing speed and reducing the amount of memory consumed. In this case, however, the process of substituting all of the lower bit planes with ‘0’ is next to the coefficient bit modeling, and thus there are many cases in which the processing time is not markedly changed, depending on the kind of bit modeling, as compared with ‘JPEG2000’.